Contamination of soil by heavy metals is an environmental problem that plagues many areas of the country and is a growing problem throughout the industrialized nations.
Most frequently, the soil surrounding industrial sites such as metal finishing and plating operations and certain manufacturing processes has become contaminated by the accumulation of heavy metals. As scientists and policy makers have come to appreciate the severity of the hazard to living organisms posed by these metals, strict environmental regulations have been put into place that govern the disposal of these wastes.
Once an area of soil is contaminated, over time these contaminants may enter streams, lakes and sources of drinking water including underground wells. For this and other reasons, in addition to controlling polluting sources, it is important to take active steps to remove dangerous contaminants from the environment. Due to the severity of the risk, persons or businesses associated with a contaminated site often face the need for a process to remove contaminants from a site in order to comply with increasingly strict pollution standards.
Compliance with strict environmental regulations is an increasingly important aspect of waste clean-up. One difficulty that arises with some existing techniques is that the treatment processes may create by-products that themselves must be treated as hazardous wastes. Ideally, an environmental remediation process would enable the user, upon completion of the process, to certify that an environmental hazard had been treated in such a way that, not only is the threat to life removed, but also that the site may be certified to be in compliance with regulations. In such case, a distinct advantage would be offered by a process that could establish that the hazardous substance has been rendered non-hazardous without producing hazardous by-products.
Also, under the laws of many states, a party who owns, leases or who once owned or leased a parcel of land may incur substantial civil and criminal liability if it is discovered that the land contains hazardous wastes. In many situations, a party, whether in or out of possession of the land, may be required to provide extensive remedial measures to remove hazardous wastes from a site even though contamination of a site is caused by materials such as heavy metals whose toxicity was unrealized or whose disposal went unregulated for decades. Furthermore, the most recently enacted environmental regulations strictly limit the quantity of metal contaminants that may be found in a soil sample and may require a party to demonstrate that the quantity of a contaminant found in a soil sample is near the natural background level. Therefore, treatment processes need to be able to offer almost complete removal of contaminants from a site before the site is considered environmentally safe.
One current practice used to clean contaminated sites involves simple removal of the contaminated soil and transfer to a certified waste disposal site. At best, this is only a partial remedy because, essentially, the contaminants are simply moved from one place to another. Additionally, soil that contains contaminants may be classified hazardous waste and therefore be subject to restrictions in transportation.
Under current law, substantial legal liability may be incurred by a party who ships toxic waste. For example, the party who ships the waste may be held liable for accidents that occur in transit and can be held vicariously responsible for the actions of unscrupulous transporters. Moreover, the physical removal, transport, and disposal of contaminated soil is dangerous to workers and can cost as much as approximately $300 per ton. Furthermore, in the near future, recent legislation has been considered that may further limit or prohibit transportation of contaminated soils to disposal sites. Thus, the need for a process to remove contaminants from soil without transporting the contaminated soil is keenly felt. Ideally, soils that are contaminated with heavy metals would be treated on-site and the bulk soil would be decontaminated and returned to the ground so as to avoid relocating large volumes of contaminated soils or having to import quantities of clean soil to make up for the quantity of hazardous soil removed.
As an alternative to physical removal of soil containing metal contaminants, soil can be chemically treated with strong acids or caustic agents to attempt to dissolve the metal contaminants. Although this method may be less expensive on a "per site" basis than physically transporting contaminated soil to a disposal site, current chemical treatment processes require construction of a specially designed treatment plant whose cost can run into the millions. More importantly from the environmental viewpoint, treatment with harsh chemicals dissolves other soil components and ignores the valuable organic properties of soil that enable soil to act as a growth medium for plants and a habitat for animals.
Those skilled in the art appreciate that decontamination of soil presents unique problems because, upon treatment to remove toxic materials, the soil should be preserved as an organic medium. For this reason, chemical reactions or techniques that might otherwise be used to treat contaminating metals would be less desirable in soil applications because the soil itself would be destroyed or further contaminated by the treatment. Despite the known drawbacks, such processes are widely used today even though they may destroy the valuable biological characteristics of the soil or actually dissolve the components of the soil entirely. Furthermore, the residues and byproducts of such chemical treatment processes could be considered hazardous wastes under state or federal regulations.
Currently, a variety of techniques exist that are capable of treating heavy metal contaminants in soil. For example, U.S. Pat. No. 5,037,240 to Sherman et al. discloses a method for introducing chemicals into soil containing any type of contaminant by sinking wick drains on the down-side of the underground water flow or directly into a contaminated water table. Using the wick drains, one can introduce a variety of remedial-treatment agents, including bacterial cultures, according to the type of contamination found. This method uses wick drains to reach the contaminants in situ and accordingly does not involve removal of soil from a site. Although this method allows one to introduce chemicals or agents to treat contaminants, unless the contaminants are in solution or are accessible to a liquid based treatment, the contaminants will not be contacted by the treating solution and will remain in the ground. Furthermore, all in situ techniques suffer from the drawback that when liquid cleansing solutions are placed into the earth, they have a tendency to disseminate unevenly. Small differences in the permeability of regions of underground soil can cause "channeling" of the treating agents thus leaving contaminated portions of a site untreated. Still further, an important problem with in situ techniques arises where a party must under government regulations certify that a site has been decontaminated to background levels. It would be extremely difficult, if not impossible, to certify that a site had been decontaminated to any certain degree. Lastly, the advantages derived from the ability to treat heavy metal contaminants by sequential treatments with different chemical agents is obviously difficult in any in-situ method.
A variety of techniques that attempt to segregate or partially isolate a contaminant are known to those skilled in the art. For example, soil screening, flotation, and other techniques are known to attempt to separate undesired components from a mixture. Also, chemical techniques for removing metals from solutions are known. For example, chelating agents are a class of molecules known to bind metal molecules and can be used to form complexes with metals in solution. Also, it is generally recognized that certain metals may be removed from an aqueous solution by electrochemical techniques.
A paper by E. Radha Krishnan, P.E. and William F. Kemner entitled "Innovative Electromembrane Process for Recovery of Lead from Contaminated Soils" discloses an electromembrane reactor that uses electric current to remove a single metal lead from a solution by electro-deposition. The preliminary steps of the process involve screening and classification, chelation of lead using EDTA, and a dewatering step, before placing a solution in an electromembrane reactor with addition of disodium carbonate for treatment. This process avoids the use of strong acids but suffers from several disadvantages that are overcome by the process of this invention.
First, as stated in the paper, pH is a very important process condition that may vary from site to site and must be controlled. Second, the electrodeposition process requires a large volume of water for the cells to function and to maintain a proper chemical balance. As the authors note, water from the electrodeposition step must be treated as a waste. The problems this causes appear to be substantial because the electrodeposition process requires a continuous inflow of water to maintain the chemical balance in the electrolyte in the electromembrane reactor. Third, the electrodeposition process is inherently slow, requires an entire installation of electrolytic cells, and may be less efficient when a variety of metals is present in a solution. Fourth, electrodeposition processes have a limit on the concentration of metals that can be removed from a solution. In other words, an electrodeposition-based process will always leave some dissolved metals remaining in solution and thus may not be capable to remove metal contaminants to background levels. Ultimately, the principal disadvantages of electrodeposition are practical ones--the need for large volumes of water and the need to transport electrodeposition reactors and the creation of by-products, particularly in the form of waste water, that are classified and must be treated as hazardous wastes.
To be most effective, a process would render the hazardous metal contaminants in soil non-hazardous by recovery and recycling and would accomplish the main goal of the environmental specialist which is to return a contaminated site to as near as possible its original condition without the practical problems caused by generating additional hazardous substances.
In light of the foregoing, it is easily appreciated that an ever-increasing need exists for an economical, portable, self-contained process that enables one to remove metal contaminants from soil, to measure the levels of contaminants in a quantity of soil to insure compliance with environmental standards, and to allow the contaminating metals to be recovered and recycled in their purified form as valuable materials. Moreover, it would be advantageous to provide a process that avoided the need to process water waste and that avoided the need to transport contaminated soil for disposal and replace contaminated soil with fresh soil. Furthermore, an ideal decontaminating chemical treatment would leave the organic quality of the soil intact.
An additional desirable feature in an environmental remediation process would be the ability to easily transport to the site at which the contamination is discovered all the equipment and materials necessary to perform the process. Also, an ideal process would be, to the extent possible, self-contained thereby requiring a minimum of equipment and material. In other words, a process that required large quantities of water or produced large quantities of waste water or other by-products would be less desirable for sites where sources of water or disposable facilities are limited or for sites that are geographically remote.